1. Field of the Invention
This invention relates to an image display apparatus provided with an automatic pedestal processing section for detecting the minimum signal level of input video signals and controlling the signal level of the video signals on the basis of the detected results. The present invention may be applied to, for example, an image display apparatus, such as a television receiver having display means in the form of a display raster or screen having a wide aspect ratio.
2. Description of the Prior Art
An image display apparatus, such as a television receiver, adapted for displaying an image by image signals or video signals by on a display means such as a Braun tube or liquid crystal display, has been known widely. With such image display apparatus, an automatic pedestal processing section for detecting the minimum signal level of the input video signals and controlling the signal level of the video signals on the basis of the detected results is provided in the image signal processing system for displaying an image with excellent picture quality by effective exploitation of the dynamic range of the display means.
Meanwhile, with the present television broadcasting system, the display screen has an aspect ratio of 4:3. In a second generation extended definition TV (EDTV), scheduled to be practiced in future, or a high quality television broadcasting system, such as so-called "high vision" broadcasting system, a widescreen having an aspect ratio of 16:9 is scheduled.
Thus it is estimated that television broadcasting in the future will be made with both the presently adopted standard aspect ratio of 4:3 and the wide aspect ratio of 16:9. For this reason, development of an image display apparatus adapted for displaying images of both types of image signals is proceeding at present.
In such image display apparatus, various display systems such as shown in FIGS. 1 to 4 are presently proposed as the system for displaying the image of the video signals with the standard aspect ratio of 4:3, using display means for the display screen having a wide aspect ratio of 16:9, as an example.
In the first system, upper and lower regions P.sub.1 and P.sub.3 of an image P.sub.N of image signals having a standard aspect ratio are cut as shown in FIG. 1 to display an image P.sub.3 on the entire display screen having a wide aspect ratio. With this first system, the image is displayed on the display screen of the wide aspect ratio by overscanning in the vertical direction of the screen without meddling with the image signals of the standard aspect ratio.
In the second system, as shown in FIG. 2, a right-hand side region A.sub.R of the display raster of the wide aspect ratio is masked and the image P.sub.N of the image signals of the standard aspect ratio is displayed in the left-hand side region of the display raster of the standard aspect ratio. With the second system, the image signals having the standard aspect ratio are compressed along the time axis to three-fourths in the horizontal direction in conformity to the difference in the aspect ratio relative to the display raster of the wide aspect ratio. Frame signals displaying the right-hand side region A.sub.R by, for example, a black tint, are annexed, and the image display is made on the display screen of the wide aspect ratio by usual raster scanning.
In the third system, shown in FIG. 3, left- and right-hand side regions A.sub.LO and A.sub.RO of the display raster having the wide aspect ratio are masked and the image P.sub.N of the image signals having the standard aspect ratio is displayed at the center of the display screen of the wide aspect ratio. With this third system, image signals of the standard aspect ratio are compressed along the time base to three-fourths in the horizontal direction, in conformity to the difference in aspect ratio relative to the display screen of the wide aspect ratio. Frame signals associated with the left- and right-hand side regions A.sub.LO and A.sub.RO are affixed and display is made on the display screen of the wide aspect ratio by usual raster scanning.
In the fourth system, shown in FIG. 4, a left-hand side region A.sub.L of the display screen having the wide aspect ratio is masked, and the image P.sub.N of the image signals of the standard aspect ratio is displayed on the right-hand side of the display screen having the wide aspect ratio. With this fourth system, the image signals of the standard aspect ratio are compressed along the time base to three-fourths in the horizontal direction, in conformity to the difference in the aspect ratio relative to the display screen of the wide aspect ratio. Masking signals associated with the left-hand side region A.sub.L are affixed and display is made on the display screen of the wide aspect ratio by the usual raster scanning.
Meanwhile, with the image display apparatus, adapted for displaying the image of the wide aspect ratio and the image of the standard aspect ratio by display means consisting of the display screen having the wide aspect ratio, a problem is raised in that, when an automatic pedestal processing section is provided for improving the picture quality of the displayed image, the masking signal having the signal level within the range of level detection by the automatic pedestal processing section is erroneously detected as being of the black level, so that the automatic pedestal processing section is erroneously actuated and hence it becomes impossible to make image display in such a manner as to take advantage effectively of the dynamic range of the display means.